Nowadays, in response to growing awareness of the need to conserve the global environment, there is a strong demand for improvements in fuel efficiency in order to reduce the amount of CO2 emissions from automobile. Accordingly, there is an active trend toward reducing the weight of an automobile body by reducing the thickness of automobile body parts through increasing the strength of a steel sheet, which is a material for automobile body parts.
Solid solution strengthening chemical elements such as Si and Mn are added in order to increase the strength of a steel sheet. However, since such chemical elements are easily oxidizable chemical elements which are more readily oxidized than Fe, the following problems exist in the case where a galvanized steel sheet or a galvannealed steel sheet is manufactured from a high-strength steel sheet as a base containing such chemical elements in large amounts.
Usually, in order to manufacture a galvanized steel sheet, a galvanizing treatment is performed after having heated and annealed a steel sheet in a non-oxidizing atmosphere or a reducing atmosphere at a temperature of about 600° C. to about 900° C. Easily oxidizable chemical elements in steel are selectively oxidized even in a non-oxidizing atmosphere or a reducing atmosphere which is generally used, are concentrated on the surface of a steel sheet, and form oxides on the surface of the steel sheet. Such oxides deteriorate the wettability between the surface of the steel sheet and molten zinc when a galvanizing treatment is performed, which results in coating defects. Wettability sharply deteriorates with an increase in the concentration of easily oxidizable chemical elements in steel, which increases occurrence of coating defects. In particular, Si significantly deteriorates the wettability between the surface of a steel sheet and molten zinc even in the case where the Si content is small, and thus Mn, which has a smaller effect on wettability than Si, is added to a galvanized steel sheet in many cases. However, since Mn oxides also deteriorate the wettability between the surface of a steel sheet and molten zinc, the problem of coating defects described above is significant in the case where the Mn content is large.
In response to such a problem, Patent Literature 1 proposes a method in which the wettability between the surface of a steel sheet and molten zinc is improved by heating a steel sheet in an oxidizing atmosphere in advance in order to rapidly form an Fe oxide film on the surface of a steel sheet at an oxidizing rate higher than a certain oxidizing rate for the purpose of preventing the oxidation of added chemical elements on the surface of the steel sheet and by performing thereafter reduction annealing on the Fe oxide film. However, in the case where the amounts of oxides on the surface of a steel sheet are large, iron oxides adhere to rolls in a furnace, which results in a problem of pressing flaws occurring on the surface of the steel sheet. In addition, since Mn forms a solid solution in an Fe oxide film, there is a tendency for Mn oxides to be formed on the surface of a steel sheet when reduction annealing is performed, which results in a decrease in the degree of the effect of the oxidizing treatment.